The present invention provides a method of making a mask arrangement including two (2) or more masks for a color cathode ray tube (CRT). Typically the masks are positioned at a small distance from the CRT's phosphor screen and are separated from each other, each aperture of each mask being arranged coaxially with a corresponding aperture of another mask over the entire effective area of all masks.
One such CRT having this type of mask structure is the mask-focusing color picture tube. In a mask-focusing color picture tube, different potentials are applied to the masks and an electrostatic lens is formed by the biased masks. The electron beam utility factor of the CRT is significantly increased compared with a convential shadow mask type color CRT. A mask-focusing color picture tube is described in Japanese Utility Model publication No. 38930/1972, and U.S. Pat. Nos. 2,971,117 and 3,398,309.
Another type of CRT which has the above described mask structure is described in Japanese Patent Publication No. 2698/1980. In that particular color CRT the mask arrangement includes two (2) masks. One mask acts as a color selection electrode and the other mask acts as a electron shield for preventing the other mask from being bombarded by electronic beams and from being deformed by a rise in temperature resulting from that bombardment.
In the both types of color CRTs, the corresponding apertures of the masks must be aligned coaxially with the electron beams. However, it is difficult to make or assemble a plurality of masks with the desired high precision of coaxial alignment. In a conventional manner, each apertured flat mask is pressed into its desired curved shape independently from the other mask(s). The masks are made of thin metal plates and have relatively large areas so that they are subject to being deformed during handling in the manufacturing process. The curvature of each mask is inevitably slightly different from that of the other masks at a given position on the masks. Therefore, it is difficult to precisely align the corresponding apertures of each mask.